This work involves four phases: a. Testing in gnotobiotic and barrier maintained rats the cariogenicity of several oral microorganisms. b. Examining the relevance of symbiotic/antagonistic relationships of Streptococcus mutans and Streptococcus sanguis isolated from the same population to the cariogenicity of these organisms. c. Evaluation of Actinomyces viscosus in the development of periodontal disease. d. Study of implantation and distribution of the various sera types of Streptococcus mutans in the mouth of gnotobiotic rats when all types are provided in a common inoculum.